Re: Propagate pathkeys from CTEs up to the outer query

I got around to looking at this finally.  I was a bit surprised by
your choice of data structure.  You made a per-CTE-item cte_paths
list paralleling cte_plan_ids, but what I had had in mind was a
per-subplan list of paths paralleling glob->subplans and subroots.
This would mean that the code for ordinary SubqueryScans would
also need to fill in that list, but surely that's a trivial cost
compared to everything else we do to prepare a subplan.  I don't
think that we have any immediate need to remember that info for
an ordinary SubqueryScan, but it seems plausible that we will
in future.  Also, I'm not sure that a Path is fully interpretable
without the associated PlannerInfo (subroot), so keeping it
beside the list of subroots seems more future-proof than dissociating
it from that.  This approach would also be more amenable to postponing
creation of the subplans, as we speculated about earlier.  (I have
no near-term desire to actually do that, but maybe someday it will
happen.)